1. Field of the Invention
The present invention relates to precision voltage reference circuits, and in particular, to precision voltage reference circuits which are compensated for variations in operating temperatures by using gain trimming techniques.
2. Description of the Related Art
Precision voltage reference circuits that operate at power supply voltages of 5 volts or less generally use circuits employing the well known bandgap principle. Many conventional bandgap reference circuit designs exist which employ one or both of BJT (bipolar junction transistor) and CMOS (complementary metal oxide semiconductor) technologies. The BJT technology has many inherent advantages. For example, the higher gain characteristics of BJTs makes them more suitable for amplifier stages with greater gain, lower noise, lower offsets and greater output drive capability. The availability of complementary BJT types (e.g., NPN and PNP) provides for gain within the .DELTA.Vbe core circuit and minimizes the contributions of amplifier noise and offset errors.
Technology for BJTs has progressed to the point where the limiting factors in the ultimate performance characteristics of the circuits have become performance shifts that occur during package assembly and package aging. While trimming techniques do exist (such as laser trimming of thin film resistors) that provide for some extreme performance ranges and resolutions, these pre-assembly trimming techniques are inadequate for overcoming performance shifts caused by the mechanical stresses of package assembly. These shifts limit the yield of the highest grade of voltage references (i.e., those with performance variations of less than 5 parts per million per degree centigrade) and, therefore, increase the cost of manufacturing.
A further major contributor to the cost of precision references is the need for characterizing each unit over several temperatures so as to guarantee performance within specified ranges of temperatures. Without temperature testing, anomalies that affect the idealized relationship between the room temperature value of the circuit output and the associated values over temperature would result in the shipping of an unacceptable proportion of defective units, i.e., units which did not maintain the desired reference voltage output within the narrow specified voltage range over the full range of operating temperatures. Moreover, temperature testing of the units in production quantities requires not only multiple handling of the units at various temperatures, but also the tracking of the individual units during such testing.
One conventional trimming technique does exist which provides for post-assembly trimming of a voltage reference, as disclosed in U.S. Pat. No. 4,751,454 (incorporated herein by reference). However, such a trimming technique provides no tracking of the output reference voltage and provides limited compensation for the complex voltage variation characteristics of the output reference voltage over temperature. Accordingly, it would be desirable to have a precision voltage reference circuit with temperature compensation that tracks the output reference voltage and has sufficiently complex tracking characteristics to compensate for the complex variation characteristics of the output reference voltage.